The present invention relates to an intermediate frequency signal processing circuit which eliminates distortion introduced by an IF (intermediate frequency) filter provided in an IF amplifying stage of an FM receiver.
A block diagram shown in FIG. 1 illustrates an IF signal processing stage. An IF signal outputted from a front end 1 is applied through an IF filter 2 to an IF amplifier 3. An output amplified in the IF amplifier 3 is detected in an FM detector 4. The IF filter 2 is required to have both a selection characteristic and a low-distortion characteristic so as to exclude an interfering signal. A Bessel-type IF filter which has precise amplitude characteristic and phase characteristic is suitable for use in the IF filter. However, the Bessel-type IF filter is not satisfactory because the improvement in the characteristic thereof is limited and thus the required characteristics are not perfectly met. In view of such a circumstance, it has been difficult to reduce distortion introduced by the IF filter.
FIG. 2 shows an FM negative feedback type IF signal processing circuit which has been practically used to improve the characteristic of the circuit shown in FIG. 1. An RF (radio frequency) signal outputted from an RF amplifier 5 is mixed with an output of a VCO (voltage-controlled oscillator) 6 in a mixer 7 and is converted into an IF signal. The latter signal is applied through the IF filter 2, the IF amplifier 3, and the FM detector 4 to an LPF (low-pass filter ) 8. The output of the LPF 8 is fed back to the VCO 6 as a control signal to control the oscillation frequency of the VCO 6. The FM detection output is frequency modulated by means of the VCO 6 and is then fed as a mixing input to the mixer 7.
With a circuit thus arranged, the frequency deviation (.+-.75 KHz) of the RF signal is compressed, for example, to .+-.7.5 KHz by the mixture with the output of the VCO 6, whereby the bandpass range of the IF signal is made narrow. As a result, the apparent bandpass range of the IF signal is extended when compared with the circuit shown in FIG. 1, and thus distortion is reduced.
However, such an FM negative feedback system is disadvantageous in that compression of the frequency deviation adversely lowers the detection efficiency, and selectivity is degraded due to simultaneous compression of a disturbing signal. Furthermore, the circuit becomes unstable and thus oscillation is liable to occur because of the use of the negative feedback system.